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Date: Thu, 9 Apr 2009 17:40:59 +0200
From: Samuel Thibault <samuel.thibault@ens-lyon.org>
Subject: Re: [Qemu-devel] buffer alignment for block backends
Message-ID: <20090409154059.GJ5457@const.bordeaux.inria.fr>
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To: Paul Brook <paul@codesourcery.com>
Cc: qemu-devel@nongnu.org

Paul Brook, le Thu 09 Apr 2009 16:38:51 +0100, a écrit :
> On Thursday 09 April 2009, Samuel Thibault wrote:
> > Anthony Liguori, le Thu 09 Apr 2009 08:11:27 -0500, a écrit :
> > > >- ide.c:ide_init2
> > >
> > > This buffer is only used when not doing DMA.  When doing DMA, we are
> > > able to do zero-copy IO so the alignment of the request depends on how
> > > the guest aligned the request.  I suspect you'll find a lot of guests
> > > that, in practice, do not align requests at 4k boundaries.
> >
> > Eeeeerrr, why shoudn't they?  Guests usually work on pages, which are
> > already aligned on 4k boundaries.
> 
> Assuming that page == 4k is going to get you into all sorts of trouble.

I'm not assuming it. I'm saying that most of the time that's the case,
and thus a heuristic that works better when that's the case will get
great benefits.

Samuel